One class of motion detector systems, also known as intrusion alarm systems, employs a sensitive receiver in conjunction with a transmitter to receive and measure an electric field. If an intruder or foreign object disturbs the electric field there results a variation in the field strength which is detected by the receiver that operates to trigger an indicator or alarm system.
Another class of detectors is a space alarm system characterized by the transmitting of energy into a specified space to be protected, or the space surrounding an object to be protected, and subsequently receiving that portion of the transmitted energy that is reflected by the surroundings. An alarm is triggered upon detection of a disturbance, i.e., a frequency change, in the reflected energy caused by an intruder or movement of an object within the area. Any frequency change of the reflected energy, as compared to the transmitted energy, will indicate movement of an intruder or an object within the area being monitored. This is the principle of operation of the well-known "Doppler" effect motion detection systems. This type of system detects a Doppler frequency shift in radiation reflected by moving objects or an intruder moving within a specified area.
Heretofore, Doppler intrusion alarm systems or motion detection systems continuously radiated (continuous wave--CW) microwave signals into the specified area and processed the reflected Doppler information through only one channel. Such systems are subject to nuisance alarms by large objects (targets), e.g., large trucks, which are not violating the specified area and are moving well outside the desired zone of protection. Such earlier available Doppler intrusion alarm systems are also plagued by nuisance alarms from innocuous vibrating reflective objects, e.g., gates or fences moving in the wind, or the rocking of an aircraft wing, which may be located within or outside the desired zone of protection.
As a result of unlimited ranging, and by way of example, previous Doppler intrusion alarm systems that were employed to protect the zone around a parked aircraft would be nuisance alarmed by normal activity such as the movement of fuel trucks or other service vehicles well outside the desired protected zone. Nuisance alarms would also be generated by such innocuous movements as the rocking of the aircraft wing due to wind. Another type installation where nuisance alarms often resulted has the specified area identified by the walls of a wooden building which are somewhat transparent to microwave energy. Trucks and other large vehicles passing around the building and metal building elements vibrating within the building are all potential sources of nuisance alarms where, as in previous motion detector systems utilizing microwave energy and the Doppler principle, a limit was not placed on the system range.